AN INNOVATIVE minipiled structure is being used to stabilise a 160-year-old railway cutting at Hooley in Surrey.
Excavated into the North Downs chalk at 50-55º, the cutting is on the Brighton-Redhill line and is between 18m and 30m deep. Up to 7m of soil and gravel lies at the top of the cutting slopes on the western side.
Recent extreme weather caused this material to become saturated and unstable, leading to a series of slope failures at the top of the cutting, threatening the railway lines as well as properties at the crest of the slope.
Fondedile Foundations was awarded the stabilisation contract by a Railtrack/ Laing joint venture in July 2001. The firm proposed stabilising the cutting using a series of vertical and horizontal reinforced concrete beams tied into the chalk using mini piles in tension and compression.
Piles are being installed directly into the chalk about two thirds of the way up the cutting sides. These will form the base of the retaining structure for the soil and gravel at the top of the slope.
The first horizontal reinforced concrete beam sits just below the gravel deposits. The amount and spacing of the horizontal and vertical beams above will vary according to the ground conditions. Netting will then be placed between the reinforced concrete members.
A 500m long scaffold working platform had to be erected in a single week-long possession at the beginning of August. This had to be quickly followed by access towers for men and materials to allow Fondedile to start piling work, which is being carried out under 'green zone' conditions that allow the railway to remain fully operational.